The preparation of styrene diene block copolymers (“SBC”) is well known. In a representative synthetic method, an initiator compound is used to start the polymerization of one monomer. The reaction is allowed to proceed until all of the monomer is consumed, resulting in a living homopolymer. To this living homopolymer is added a second monomer that is chemically different from the first. The living end of the first polymer serves as the site for continued polymerization, thereby incorporating the second monomer as a distinct block into the linear polymer. The block copolymer so grown is living until terminated. Termination converts the living end of the block copolymer into a non-propagating species, thereby rendering the polymer non-reactive towards a monomer or coupling agent. A polymer so terminated is commonly referred to as a diblock copolymer. If the polymer is not terminated the living block copolymers can be reacted with additional monomer to form a sequential linear block copolymer. Alternatively, the living block copolymer can be contacted with multifunctional agents commonly referred to as coupling agents. Coupling two of the living ends together results in a linear triblock copolymer having twice the molecular weight of the starting, living, diblock copolymer. Coupling more than two of the living ends together results in a radial block copolymer architecture having at least three arms.
One of the first patents on linear ABA block copolymers made with styrene and butadiene is U.S. Pat. No. 3,149,182. These polymers in turn could be hydrogenated to form more stable block copolymers, such as those described in U.S. Pat. Nos. 3,595,942 and Re. 27,145. Selective hydrogenation to remove the C═C moieties in the polydiene segment of such polymers is critical in preparing block copolymers with good thermal and chemical resistance, particularly resistance to oxidative degradation.
In the past functionality was added to the block copolymer in order to modify the properties of the block copolymer and increase its ability to react with other monomers and polymers. One of the ways to add functionality to such polymers is carboxylation. Carboxylated block copolymer are disclosed in a number of issued patents including U.S. Pat. Nos. 4,797,447; 4,868,243; 4,868,245; 5,002,997; 5,209,862; and 5,218,033. Still another type of block copolymers that have been carboxylated in the past are selectively hydrogenated styrene/butadiene block copolymers that have a controlled distribution interior block containing both styrene and butadiene, as opposed to the normal block copolymers that just contain butadiene in the interior block. Such block copolymers are disclosed in Published U.S. Patent Application Nos. 2003/0176582 and 2005/0137349, U.S. Pat. No. 7,138,456, as well as PCT Published Application WO 2005/03812.
In the carboxylated block copolymers disclosed above, invariably the outer (hard) blocks are carboxylated due to the presence of styrene in the outer blocks. This means that upon exposure to water, hydration of the hard domains in the material will result in plasticization of those domains and significant softening. This softening of the hard domains results in a marked decrease in the mechanical integrity of membranes prepared from these block copolymers. Thus, there is a risk that when exposed to water any structure supported by these prior art carboxylated block copolymers will not have sufficient strength to maintain its shape. Hence, there are limits to how to use such a block copolymer and limits on its end use applications. In particular, what is needed is a semi-permeable membrane with high water transport properties that maintains sufficient wet strength for a wide variety of applications.